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  Java Practices -> Avoid clone
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<div class='page-title'>Avoid clone</div>

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<br>Avoid implementing <tt><a href="http://java.sun.com/javase/6/docs/api/java/lang/Object.html#clone()">clone</a></tt>.
<ul>
<li>
<tt>clone</tt> is very tricky to implement correctly in all circumstances,
nearly to the point of being pathological</li>

<li>
the importance of copying objects will always remain, since object fields
often need to be <a href="TopicAction79f1-2.html">defensively copied</a></li>

<li>
<a href="TopicAction6f4f-2.html">copy constructors</a> and <a href="TopicAction8d16-2.html">static
factory methods</a> provide an alternative to
<tt>clone</tt>, and are <i>much</i>
easier to implement</li>
</ul>
<em>If you need to extend a superclass that implements <tt>clone</tt></em>, then 
your subclass must implement <tt>clone</tt> as well. The quickest solution is 
for your subclass to simply throw an exception.

<p><b>Example</b>
<p>This example shows a superclass with a typical implementation of <tt>clone</tt>,
and a subclass which has disabled its <tt>clone</tt> method.
<br>
<PRE>

<span class='keyword'>import</span> java.util.Date;

<span class='keyword'>public</span> <span class='keyword'>abstract</span> <span class='keyword'>class</span> Fruit <span class='keyword'>implements</span> Cloneable {

  <span class='keyword'>public</span> Fruit( String aColour, Date aBestBeforeDate ) {
    <span class='keyword'>super</span>();
    fColour = aColour;
    <span class='comment'>//defensive copy needed for this mutable object
</span>    fBestBeforeDate = <span class='keyword'>new</span> Date( aBestBeforeDate.getTime() );
  }

  <span class='keyword'>public</span> <span class='keyword'>abstract</span> <span class='keyword'>void</span> ripen();

  <span class='keyword'>public</span> String getColour() {
    <span class='keyword'>return</span> fColour;
  }

  <span class='keyword'>public</span> Date getBestBeforeDate() {
    <span class='comment'>//return defensive copy of this mutable object
</span>    <span class='keyword'>return</span> <span class='keyword'>new</span> Date ( fBestBeforeDate.getTime() );
  }

  <span class='comment'>/**
  * Implement clone as follows
  * &lt;ul&gt;
  * &lt;li&gt;the class declaration "implements Cloneable" (not needed if already
  * declared in superclass)
  * &lt;li&gt;declare clone method as public
  * &lt;li&gt;if the class is final, clone does not need to throw CloneNotSupportedException
  * &lt;li&gt;call super.clone and cast to this class
  * &lt;li&gt;as in defensive copying, ensure each mutable field has an independent copy
  * constructed, to avoid sharing internal state between objects
  * &lt;/ul&gt;
  */</span>
  <span class='keyword'>@Override</span> <span class='keyword'>public</span> Object clone() <span class='keyword'>throws</span> CloneNotSupportedException {
    <span class='comment'>//get initial bit-by-bit copy, which handles all immutable fields
</span>    Fruit result = (Fruit)<span class='keyword'>super</span>.clone();

    <span class='comment'>//mutable fields need to be made independent of this object, for reasons
</span>    <span class='comment'>//similar to those for defensive copies - to prevent unwanted access to
</span>    <span class='comment'>//this object's internal state
</span>    result.fBestBeforeDate = <span class='keyword'>new</span> Date( <span class='keyword'>this</span>.fBestBeforeDate.getTime() );

    <span class='keyword'>return</span> result;
  }

  <span class='comment'>/// PRIVATE ////
</span>
  <span class='comment'>/**
  * Strings are always immutable.
  */</span>
  <span class='keyword'>private</span> String fColour;

  <span class='comment'>/**
  * Date is a mutable object. In this class, this object field is to be treated
  * as belonging entirely to this class, and no user of this class is
  * to be able to directly access and change this field's state.
  */</span>
  <span class='keyword'>private</span> Date fBestBeforeDate;
} 
</PRE>
<br>
Here is the subclass, with its <tt>clone</tt> method disabled.
<br>
<PRE>

<span class='keyword'>import</span> java.util.Date;

<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> Apple <span class='keyword'>extends</span> Fruit {

  <span class='keyword'>public</span> Apple( String aColour, Date aBestBeforeDate ) {
    <span class='keyword'>super</span>( aColour, aBestBeforeDate );
  }

  <span class='keyword'>public</span> <span class='keyword'>void</span> ripen() {
    <span class='comment'>//empty implementation of abstract method
</span>  }

  <span class='comment'>/**
  * The Apple subclass does not support clone.
  */</span>
  <span class='keyword'>@Override</span> <span class='keyword'>public</span> <span class='keyword'>final</span> Object clone() <span class='keyword'>throws</span> CloneNotSupportedException {
    <span class='keyword'>throw</span> <span class='keyword'>new</span> CloneNotSupportedException();
  }
} 
</PRE>
<br>
<br>
<br>

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<div class='topic-section'>See Also :</div>
<div class='main-body'>
 
  
  <a href='TopicAction6f4f-2.html?Id=12'>Copy constructors</a> <br>
 
  
  <a href='TopicAction79f1-2.html?Id=15'>Defensive copying</a> <br>
 
  
  <a href='TopicAction8d16-2.html?Id=21'>Factory methods</a> <br>
 
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